1. Field of the Invention
The present invention relates to a semiconductor integrated circuit having an initiating circuit for surely initiating or re-initiating a circuit when the circuit is to be initiated or when the circuit operates abnormally; a test method for testing the semiconductor integrated circuit; and an electronic information device including the semiconductor integrated circuit, such as a digital camera, a digital video camera, a mobile phone device having a camera, a television device and a monitor device of a personal computer.
2. Description of the Related Art
Some semiconductor integrated circuits such as ICs and LSIs may include an analog circuit such as a constant voltage circuit and a band gap reference voltage circuit. For the semiconductor integrated circuits including an analog circuit, an initiating circuit is required for stably operating the analog circuit portion and surely initiating or re-initiating a semiconductor integrated circuit, when the circuit is to be initiated (e.g. when the power supply is turned on) or when the circuit operates abnormally. This will be described below with reference to FIG. 5.
FIG. 5 is a circuit diagram showing an exemplary structure of a main part of a conventional constant voltage circuit.
As shown in FIG. 5, the conventional constant voltage circuit has a transistor 61 and a constant current circuit 62 which define an emitter follower circuit at the output stage.
The transistor 61 functions as a buffer amplifier for the constant voltage obtained from collector current of the transistor 64 by the diodes 68-71.
The constant current circuit 62 includes PNP transistors 63 and 64, an NPN transistor 65, diodes 68-71 and a resistor 67. The transistors 63 and 64 define a current mirror circuit, which positively feedbacks current In2 to a base of the transistor 65 via the diodes 68 and 69. The current In2 has approximately the same value as collector current Ic4 of the transistor 65 which is connected in series to the transistor 63. The constant current circuit 62 further includes an initiating circuit for surely initiating a circuit at a desired stable operation point when the power supply (the voltage Vcc1) is turned on (i.e. when the circuit is initiated) by passing current Ip1 through an initiating resistor 72.
There is a possibility that the initiating resistor 72 could be disconnected due to an adhesion of dust during a production process, for example. However, even if the initiating resistor 72 is disconnected, the semiconductor integrated circuit may be initiated due to parasitic capacitance and the like.
For example, in a case where the constant voltage circuit shown in FIG. 5 is manufactured on a monolithic integrated circuit, it is assumed that the state of the initiating resistor 72 becomes open due to dust during the production process of the integrated circuit.
In this case, due to parasitic capacitance 66 (indicated by a broken line in FIG. 5) existing between the base of the transistors 63 and 64 and ground GND, transient current In1 flows when the power supply is turned on. The transistor 65 is given In 2≈In1 current as a base current by a current mirror circuit defined by the transistors 63 and 64. When the DC current amplification ratio of the transistor 65 is hFE65, the transistor 63 is given hFE65×In2 collector current by the transistor 65. As a result, the transistor 65 is normally biased by a positive feedback loop defined by the transistors 63 and 64 and the diodes 68 and 69. Accordingly, the constant current circuit 62 can be initiated regardless of the state of the initiating resistor 72.
Even if the initiating resistor 72 is found to be open (is disconnected) during a shipping test of the semiconductor integrating circuit having the constant voltage circuit, as described above, the transistor 65 may be normally biased and may be initiated. Therefore, it is impossible to completely remove a product including the initiating resistor 72 which is open (is disconnected). This allows a shipment of a product including the initiating resistor 72 which is open (is disconnected); thereby causing the possibility of a problem that the circuit of a shipped product cannot be initiated.
The probability that the initiating resistor 72 is disconnected is extremely low. However, if the product including the disconnected initiating resistor 72 goes onto the market, the reliability of the semiconductor integrated circuit is greatly reduced. Therefore, it is necessary to surely determine whether or not the initiating circuit operates normally during the shipping test and to prevent the product including the disconnected initiating resistor 72 from going onto the market.
Conventionally, in order to perform a test for determining whether or not the initiating circuit operates normally, inspection pads, drawn from the initiating circuit of the semiconductor integrated circuit, are required.
For example, a method for determining whether or not a resistor is open by providing an inspection pad on one end of the resistor for initiating the constant voltage circuit, is proposed in Reference 1. For example, in the constant voltage circuit shown in FIG. 5, a test is made using an inspection pad 73 which is drawn from one end of the initiating resistor 72.
The inspection pad 73 occupies a rectangular area having one side measuring about 100 μm to 300 μm (for example, see Reference 2).    Reference 1: Japanese Laid-open Publication No. 7-50531    Reference 2: Japanese Laid-open Publication No. 2000-277621
Similarly, in the prior art disclosed in Reference 1, the inspection pad occupies a large area on the semiconductor integrated circuit. The rectangular area having one side measuring about 100 μm to 300 μm is usually required with reference to Reference 2. Therefore, there exists a problem that the area of the inspection pad greatly reduces an area of the region on which the effective elements can be formed.
Further, the conventional inspection pad for an initiating circuit is not available for connection to terminals of a package, but only available for proving (e.g. testing) by an analog tester during a wafer test. This is because malfunctions may occur in the initiating circuit when extraneous noise or the like is input from the terminal of the package when an inspection pad is connected to the terminal of the package. Therefore, the determination whether or not the initiating circuit operates normally is conventionally required in a wafer test prior to packaging the initiating circuit. This is an obstacle in the case of omitting the wafer test to reduce testing costs and making a test prior to shipping but after packaging.
The present invention is intended to solve the conventional problems described above. One purpose of the present invention is to provide a semiconductor integrated circuit which enables the incorporation of a test determining whether or not the initiating current generating section operates normally for initiating a circuit into part of the logic test, and which enables the elimination of the inspection pads for the initiating circuit; a test method for testing the semiconductor integrated circuit; and an electronic information device including the semiconductor integrated circuit, such as a digital camera, a digital video camera, a mobile phone device having a camera, a television device and a monitor device of a personal computer.